Add Worker interface and rename Worker to DefaultWorker

[KengoWada]

Proposed changes

This update refactors the example to use the new DefaultWorkerFactory, reflecting the recent changes to support customizable workers via the Worker interface.

Related issue(s)

Fixes #9

Checklist

Please ensure that the following checks have been performed:

• I have read and followed the project's contributing guidelines.
• My code follows the project's coding style.
• I have added or updated relevant documentation.
• I have added or updated tests (if applicable).
• All new and existing tests pass.
• My changes do not introduce any new warnings or errors.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 33.33333% with 16 lines in your changes missing coverage. Please review.

Project coverage is 40.54%. Comparing base (164be30) to head (3d3ea1a).

Files with missing lines	Patch %	Lines
manager.go	0.00%	16 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #11      +/-   ##
===========================================
- Coverage    41.95%   40.54%   -1.42%     
===========================================
  Files            4        4              
  Lines          143      148       +5     
===========================================
  Hits            60       60              
- Misses          83       88       +5     

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